1. Field of the Invention
This invention relates to a liquid crystal display panel and a process for producing the same. More particularly, this invention relates to an alignment division type liquid crystal display panel, wherein different alignment regions are formed in a pixel region, and a process for producing the same. The term "alignment division type" used herein refers to such a construction that in order to provide high viewing angle characteristics, an alignment film provided on the inner wall of the substrate is divided into minute domains so that the minute domains are different from each other in alignment state of the liquid crystal.
2. Description of the Related Art
In recent years, there is an ever-increasing demand for an active matrix type color liquid crystal display panel. The increase in the demand has led to diversification of requirements for liquid crystal display panels per se. Among others, an improvement in viewing angle characteristics is strongly desired in the art. Specifically, in such liquid crystal panels, the contrast between light and dark of an image unfavorably varies depending upon the angle at which the observer views the image plane, so that the elimination of this problem (this phenomenon being generally recognized as viewing angle characteristics of liquid crystal panels) is desired in the art. For example, when a liquid crystal panel, which has been subjected to alignment treatment, is viewed obliquely from above the panel, the transmittance is remarkably lowered even by the application of a very low voltage and again increases with increasing the voltage. On the other hand, when the liquid crystal panel is viewed obliquely from below the panel, the transmittance does not decrease with increasing the voltage, so that the display becomes relatively bright even if a black display is contemplated.
A current technique which has attracted attention for providing high viewing angle characteristics is an alignment division technique where one pixel region is divided into different alignment domains. For example, Japanese Unexamined Patent Publication (Kokai) No. 63-106624 proposes that one pixel is divided into two domains different from each other in alignment direction of liquid crystalline molecules to combine viewing angle characteristics in the case of viewing obliquely from above the panel with the viewing angle characteristics in the case of viewing obliquely from below the panel, so that the overall viewing angle characteristics can be improved.
The alignment state of the conventional liquid crystal display panel of this type is shown in FIG. 1. In this drawing, the area of one pixel alone is shown for facilitating the understanding, and the one pixel is divided into two domains A and B different from each other in alignment state of liquid crystalline molecules. Light is incident upon one substrate (in this case, the substrate upon which light is incident being designated as "lower substrate") and passed through a liquid crystal sandwiched between the substrates and comes out through the other substrate (upper substrate). An observer is assumed to view the liquid crystal panel from above the upper substrate. The rubbing direction of the alignment film of the lower substrate is indicated by an arrow R.sub.L, and the rubbing direction of the alignment film of the upper substrate is indicated by an arrow R.sub.U.
In the domain A shown in FIG. 1, the rubbing direction R.sub.L of the alignment film of the lower substrate is 45.degree. towards the left top, while the rubbing direction R.sub.U of the alignment film of the upper substrate is 45.degree. towards the left bottom. In such an alignment treatment, viewing angle characteristics correspond to the case where the liquid crystal panel is viewed obliquely from above the panel. In this case, the application of a slight voltage results in a markedly lowered transmittance, and the transmittance again increases with increasing the voltage. The viewing angle characteristics are indicated by a double arrow in the drawing. On the other hand, in the domain B, the rubbing direction R.sub.L of the alignment film of the lower substrate is 45.degree. towards the right bottom, while the rubbing direction R.sub.U of the alignment film of the upper substrate is 45.degree. towards the right top. In such an alignment treatment, the viewing angle characteristics are opposite to those in the domain A. When minute domain A and minute domain B different from each other in alignment treatment are provided adjacently to each other, the viewing angle characteristics become such that the sum of the high-transmittance viewing angle characteristics and the low-transmittance viewing angle characteristics is divided by 2. In this case, in both cases where the liquid crystal panel is viewed from above the panel and below the panel, the resultant viewing angle characteristics become close to the viewing angle characteristics in the case where the liquid crystal panel is viewed from the front on the whole.
FIG. 2 is a cross-sectional view of a liquid crystal panel 1 having minute region A and minute region B. As shown in the drawing, the liquid crystal panel 1 comprises a TFT substrate 16 as a lower substrate, a CF substrate 18 as an upper substrate and a liquid crystal 20 sandwiched between these substrates. The lower substrate 16 is provided with a transparent pixel electrode 5 and an alignment film 6, and the upper substrate 18 is provided with a transparent common electrode 7 and an alignment film 8. In FIG. 1, the liquid crystal molecule in minute domain A is indicated as rising obliquely towards the left top, while the liquid crystal molecule in minute domain B is indicated as rising obliquely towards the right top. The above-described viewing angle characteristics occur based on the direction of tilt of the liquid crystalline molecule.
In FIG. 2, the alignment films 6 and 8 each comprise a two-layer structure comprising a lower alignment material layer 9,4 and an upper alignment material layer 2,3. The upper alignment material layer 2,3 are patterned by lithography so as to form minute portions respectively corresponding to minute domains A and B, and the lower alignment material layers 9,4 are exposed from opening portions adjacent to the respective minute portions. The upper alignment material layers 2,3 comprising minute portions are alternately provided on the side of the upper substrate 18 and on the side of the lower substrate 16. Specifically, in minute domain A, the upper alignment material layer 2 on the side of the lower substrate 16 faces the lower alignment material layer 4 on the side of the upper substrate 18, while in minute domain B, the upper alignment material layer 3 on the side of the upper substrate 18 faces the lower alignment material layer 9 on the side of the lower substrate 16. In this case, in order to provide a liquid crystal panel 1 having minute domains A and B different from each other in alignment state of the liquid crystal, the alignment films 6 and 8 of the respective substrates should be rubbed for each of minute domains A and B. In rubbing, a mask is used for selective rubbing. In minute domain A, the upper alignment material layer 2 on the side of the lower substrate 16 is rubbed in a direction indicated by an arrow R.sub.L of domain A in FIG. 1, while the lower alignment material layer 4 on the side of the upper substrate 18 is rubbed in a direction indicated by an arrow R.sub.U of domain A in FIG. 1. This is true of minute domain B.
The liquid crystal display panel 1 is produced according to steps shown in the flow sheet of FIG. 3. After a substrate is introduced, a lower alignment material and an upper alignment material are successively coated on the inner wall of the substrate to form alignment films. Subsequently, a suitable resist material is coated for patterning the upper alignment material layer. After the formation of the resist film, a series of steps comprising selective exposure, development-etching and removal of the resist are carried out. After the formation of alignment films each patterned to a desired form, the alignment films are rubbed, a spacer is provided by spreading, a liquid crystal is introduced, and a polarizing plate is laminated, thereby completing an LC panel.
The construction, behavior and the like of the conventional active matrix type color liquid crystal display panel could be easily understood from the above description with reference to FIGS. 1 and 2. Further, in the case of the active matrix drive, a gate bus line C and a drain bus line (not shown) are provided on the lower substrate 16 having a pixel electrode 5. In the interface of minute domains A and B different from each other in alignment state, liquid crystalline molecules tilted in directions opposite to each other come into contact with each other, so that the alignment of the liquid crystal is distorted and the disclination occurs. This is likely to cause light to be leaked along the borderline. For this reason, the gate bus line C is designed to be positioned on the boundary between minute regions A and B so that the gate bus line C can shield light leaked due to the disclination, thereby obtaining a good display.
However, in the conventional construction as shown in FIG. 2 wherein the gate bus line C is designed to be positioned in the interface of minute domains A and B, the upper alignment material layer 2 at its end 2a on the side of the lower substrate 16 and the upper alignment material layer 3 at its end 3a on the side of the upper substrate 18 are formed at a position which overlaps with the gate bus line C. This causes the liquid crystal 20 to be electrolyxed to generate an ion, which is likely to deteriorate the liquid crystal 20 and, hence, often lowers the performance of the liquid crystal panel 1.
One of the reasons why the liquid crystal 20 is electrolyzed is that the liquid crystal 20 is in contact with the upper alignment material layer 2 and the lower alignment material layer 4 different from each other in material. The electrolysis occurs based on the same principle as the electrolysis in the case where two electrode pieces different from each other in material are introduced into a solution. Another reason is that the liquid crystal 20 receives direct current during driving. The gate bus line C comprises many gate lines, for example, 400 lines, which are successively scanned. Each gate line C has a plus value for 1/400th of the scan time and a constant minus value for 399/400th of the scan time. Therefore, it can be said that direct voltage is always applied to the liquid crystal 20 between the gate bus line C and the common electrode 7. In FIG. 2, a potential difference v.sub.1 occurs between the upper alignment material layer 3 and the lower alignment material layer 2 which face each other above the gate bus line C, and on the side of the lower substrate 16, a potential difference v2 occurs between the upper alignment material layer 3 and the lower alignment material layer 2. In particular, the presence of a leak defect in a gate insulating film results in an increased direct voltage applied to the liquid crystalline layer and the alignment film. Further, when two gate bus lines C are arranged parallel to each other, the potential of the space between the two gate bus lines C becomes the same as that of the gate bus line C.